1. Field of the Invention
The invention relates to a moire reducing apparatus for reducing so-called raster moire which is caused by interference between scanning lines and phosphors arranged in a vertical direction and capable of being luminous without being blocked by a shadow mask in a display apparatus utilizing a CRT (cathode-ray tube) such as a television receiver or the like.
2. Related Art of the Invention
A conventional moire reducing apparatus is described in, for example, Japanese Patent Publication (Kokai) No.HEI5-236291.
As shown in FIG. 33(A) the conventional moire reducing apparatus comprises: a frequency divider circuit 321 which divides the frequency of the vertical synchronizing signal so as to double the period and outputting a control signal; a switching circuit 322 which controls a shift amount of scanning lines in a vertical direction in accordance with the control signal from the frequency divider circuit; a vertical deflection circuit 323 which generates a deflection waveform, i.e., a so-called saw-tooth wave used for deflecting electron beams in the vertical direction, in accordance with the vertical synchronizing signal; a vertical coil which deflects the electron beams in the vertical direction; a correcting capacitor which conducts the S correction to correct the deflected amount of the electron beams so as to coincide with the curvature in the vertical direction; and a resistor which adds a DC current used for shifting the scanning lines in each field.
The operation of the conventional moire reducing apparatus having the above-described configuration will be described. The vertical synchronizing signal is input into the frequency divider circuit. The frequency divider circuit outputs the control signal having a period which is two times the vertical period to the switching circuit. The switching circuit is turned on or off (opened or closed) in accordance with the input control signal. When the switching circuit is in the on state, a constant current is supplied to the capacitor via the resistor. When the switching circuit is in the off state, any current is not supplied to the capacitor. See FIG. 33(B).
As described above, when the switching circuit is in the on state, that is, in the second field, a constant voltage is superposed on the capacitor for S correction via the resistor, so that scanning lines in the second field are shifted. As a result, the scanning lines in the second field can be shifted over the entire screen. See FIG.(C).
In this manner, the deflection current flowing through the vertical deflection coil is varied for each field, and the display position on the screen is vertically shifted for each vertical period, so that the density (contrast) of moire can be weakened and raster moire can be reduced.
In such a conventional moire reducing apparatus, however, the shift amount of scanning lines required for reducing moire must be manually adjusted in the case where the moire wavelength at which moire is visually noticeable is varied because of variation in the number of scanning lines per unit length. For example, such a case occurs when some variation is caused in the heights of display screens during production, when the user changes the height of the displayed image-on the screen, or when the system of the input signal is changed. This produces a problem in that an optimum moire reducing effect cannot be automatically attained so that the image quality is deteriorated.
In the case where there is a so-called deflection distortion such as the pincushion distortion on the display screen, the optimum shift amount of scanning lines is varied depending on a position on the screen. This produces a problem in that moire cannot be uniformly reduced over the entire screen.